The present invention relates to synchronous machines, and more particularly to a method of keeping a synchronous machine within a stable operating zone during large transient voltage excursions.
Synchronous machines are rotating electromechanical machines that can be used as either motors or generators. Synchronous machines are commonly used as generators that are rotated by steam or gas turbines, so as to be used in power systems that are part of the power supply grid.
Synchronous machines have two mechanical parts, i.e., a rotor and a stator. They also have two electrical parts, i.e., a field source and an armature winding. The field source is typically located on the rotor of the machine, while the armature winding is typically located on the stator of the machine. The armature winding may be a three phase winding.
The field source produces a magnetic field with magnetic flux that interacts with the armature winding, so as to induce an AC (alternating current) voltage in the armature winding. The field source can be either permanent magnets or a field winding with DC (direct current) flowing through it. Permanent magnets are commonly used in small machines, while field windings are commonly used in large machines. A field winding produces a magnetic field as a result of the DC current flowing through it. The field excitation in the rotor field winding by the DC field current is constant in strength and rotates around the machine at the speed of rotation of the rotor by a prime mover. The magnitude of this field excitation and the magnetic field produced by it is directly proportional to the DC field current, as long as the magnetic circuit of the rotor and stator windings is not saturated.
When a synchronous machine operates as a generator, the machine's rotor is turned by external prime mover, such as a mechanical shaft driven by a gas or steam turbine. When the DC field current flows through the field winding rotating with the machine's rotor, the rotating field winding produces a rotating magnetic field. This rotating magnetic field induces AC voltage within the stator armature winding. When the AC voltage causes an AC current to begin to flow through the three phase armature winding, a magnetic field is then created that rotates at the same speed as the magnetic field created by current flowing through the rotating field winding on the rotor, which field is rotating at the synchronous speed of the machine. Thus, the rotating magnetic field created by the rotating field winding induces a three-phase voltage within the three-phase stator winding. The stator windings are the windings where the main electromotive force (EMF) or voltage in a generator is induced.
In a synchronous generator, “load angle” δ is defined as the angle between the electromotive force (EMF) induced in the generator (E) and the generator's terminal voltage (V). “Load angle” is also defined as the angle between the rotating magnetic field created by the rotor field winding and the rotating magnetic field induced by the stator armature. For a synchronous generator, the rotor magnetic field rotates at synchronous speed and the rotating magnetic field is created in the stator armature.
The two fields are not fully aligned. Typically, the rotor magnetic field lags the rotating stator field. This lagging is expressed in an angle that is the load angle. The load angle for a synchronous generator will vary as the generator moves from a no load condition to a load condition.
Power factor is defined as the cosine of the angle between current and voltage. Power factor is also the ratio of the real power delivered to a load to the apparent power delivered to the load. Apparent power is the product of RMS (root mean square) current and RMS voltage.
Load angle is important in maintaining the stability of a generator. If the load angle exceeds ninety degrees (90°), the generator becomes unstable. This may happen when a sudden change in a large load occurs or when a fault on the power grid is sustained for a long time. More recently, and in the future, an increase in the use of renewable energy sources may affect the ability of large synchronous machines connected to the power grid to stay synchronized during large voltage excursions. A synchronous generator operates in a lagging or unity power factor mode due to an inductive load. With growing renewable energy sources, the requirement to operate machines at leading power factors might increase to maintain system voltage, near nominal values.
The transient stability of a synchronous machine is largely defined by the operating point of the machine on its load angle curve. Power system stability depends on the clearing time for a fault on the transmission system. Slower fault clearing allows the rotor to accelerate so far along the power curve and could make a synchronous machine become de-synchronized. The ability of the machine to stay synchronized during a large transient is defined by the machine's operating load angle, i.e., the angle between the rotating magnetic field of the rotating rotor and the rotating magnetic field induced in the stator armature. The enhanced field excitation control feature of the present invention provides a control on machine load angle, with a capability to ensure minimum transient and dynamic stability margins.